Earths galore

The Kepler mission team announced exciting new results on 7 January 2013, at the 221st meeting of the American Astronomical Society. As I’ve explained in other posts (such as this one here), the team aims to detect exoplanets by staring at more than 150,000 stars in a fixed part of the sky. The Kepler telescope looks for regular dips in the brightness of these stars, variations that might be caused by the presence of a transiting planet.

The technique used by the Kepler team has been tremendously successful. On 7 January Christopher Burke, a Kepler scientist at the SETI Institute, announced the discovery of 461 new planetary candidates. So, as of the time of writing, Kepler has found 2740 potential planets orbiting 2036 stars. This is a hugely impressive number, when you consider that it wasn’t such a long time ago that people were debating whether exoplanets existed at all and, if they did, whether it would be possible to detect them.

The sizes of the planetary candidates discovered by Kepler (Credit: NASA/Kepler)

The AAS presentation that really caught the attention of the world’s press, however, was that given by Francois Fressin of the Harvard-Smithsonian Center for Astrophysics. Fressin has tried to estimate the fraction of stars that possess of Earth-sized planets, based on the Kepler data. Kepler will inevitably see only a small fraction of exoplanets because the transit technique only picks up those planetary systems in which the orbital plane is more or less side on to our view. If the orbital plane is slanted by more than a few degrees to our line of sight, the planets we won’t see the planets transit and there’ll be no drop in brightness. On the other hand, Kepler is seeing regular brightness fluctuations that are not due to transits; for example, a non-variable star might be extremely close in our line of sight to a regular variable, which would give a similar signal to a transit. After Fressin had corrected for both these effects he arrived at the following estimate: one in six stars host an Earth-sized planet. If that is indeed the case, there are about 17 billion Earth-sized planets in our Galaxy and it’s certain that some of these will be in the habitable zone.

If we’re looking to explain the Fermi paradox, we now know that we can’t invoke a paucity of Earths!